During the construction of residential or commercial buildings, doors are placed in interior walls by first forming a door frame in the wall before the wall has been covered in drywall or some other covering material. Then after the wall is covered in drywall or some other covering material, a door jamb is installed against the door frame and the door can be hung in the installed door jamb.
Framing the door is still commonly done from scratch. When an interior wall is to be installed, a stud wall is constructed. This stud wall is typically constructed of a 2×4 sized piece of lumber that will run along the floor (the “bottom plate”), a series of 2×4 sized pieces of lumber extending upwards from the bottom plate at regular intervals (the “studs”) and a 2×4 sized piece of lumber running along the tops of the studs (the “top plate”). A second stud can be provide running along the top plate to form a double top plate. The door frame is constructed in this stud wall in the location where a door is desired. A carpenter (framer) will typically measure the space where the door will be in the stud wall and then cut out this space. Then he or she will cut studs for either side of the door and nail these studs between the bottom plate and the top plate of the stud wall to form a pair of outer studs. With an opening formed between the two outer studs, the carpenter will cut two more studs to the height of the door frame and install these studs along the outer studs to form inner studs. With these two inner studs in place, the carpenter then must form a header for the door. The carpenter can cut a stud so that it is the width of the opening formed by the outer studs and install this stud over the top of the inner studs. Additional smaller studs (“cripple studs”) can be installed between the stud installed above the door opening and the top plate so that the cripple studs run vertically between the two studs on the top of the door framed opening and the top plate. Alternatively, the carpenter can form the header by cutting to 2×12 pieces of lumber to the width of the door opening formed by the outer studs, placing spacers in between these 2×12 sized pieces of lumber so that they are the same thickness as the studs and nailing this above the inner studs to form the header. The 2×12 sized pieces of lumber are positioned so that their width is positioned vertically above the door opening.
To hang the door in the door frame, the door jambs can be installed in the frame and the door hung from the door jambs.
Typically, the door frame is done all by the carpenter's measurements on site. The carpenter must measure the width of the opening he or she wants for the door frame and try to take into account the extra space needed to accommodate the door jambs. Additionally, the carpenter must take into account the width of the outer and inner studs and leave room to accommodate their width in addition to the jambs that will eventually be installed within the door frame to hang the door. Because each of the studs forming the door frame are cut for that particular door frame, any cutting error can result in an opening that is too wide (or high) resulting in a lot of additional work to be done by the finishing carpenter that has to hang the door or too narrow (or low) requiring the finishing carpenter to try and widen (or heighten) the door after the fact to get the door to fit in the frame.
With the lumber for each door frame being cut on the spot for each door frame and sized by measurements of the specific spot a lot of human error can be introduced. While a carpenter can get their frames sized properly most of the time, even the odd miss-sized door frame can add significant time and complications to hanging the door. This problem can be exasperated when less experienced carpenters are creating the door frames or construction is hurried.